Method of producing non-fogging films



Sept. 20, 1966 1.. E. AMBORSKI 3,274,091

METHOD OF PRODUCING NON-FOGGING FILMS Filed Sept. 18, 1962 POLYMER FILMT0 SPARK GENERATOR ORGANIC VAPOR-+=== INVENTOR LEONARD E. AMBORSKI BYzfiz/gf ATTORNEY United States Patent 3,274,091 METHOD OF PRODUCINGNON-FOGGING FILMS Leonard Edward Amber-ski, Buffalo, N.Y., assignor toE. I. du Pont de Nemours and Company, Wilmington, DeL, a corporation ofDelaware Filed Sept. 18, 1962, Ser. No. 224,399 6 Claims. (Cl. 204165)This invention is concerned with polymeric films and more particularlywith the surface modification of certain polymeric films to inhibit thetendency of such films to collect moisture on their surfaces when incontact with atmospheres of high moisture content.

In the development of organic polymeric films for the Wrapping ofvarious articles a major effort has been directed toward providing filmswith low permeability to avoid moisture loss from the article orcontamination of the article by undesired ambient vapors and withproviding packages that are durable to avoid product loss duringcustomer handling and the like. An attendant problem in the use of suchfilms, especially for the wrapping of products of high moisture content,is the tendency of moisture within the package to condense as dropletsor as a fog on the inside surface of the package which renders, as aresult, the package unattractive and tends to obscure the view of theproduct being displayed, thus defeating, in large measure, the purposefor using a transparent wrapping material. In still other applicationsof films such as in solar stills, for example, it is important to theattainment of high efficiency that the distillate moisture vapor beingcondensed collect on the condensing surface of the still as a smoothlayer and not as droplets.

It has been recognized for some time that if the surface of a film couldbe made more wettable the tendency of moisture vapor to collect asdroplets would be minimized and moisture collected on the surface wouldbe disposed essentially as a transparent continuous film. The varioustreatments such as flame, electrical discharge, or chemical treatmentsthat have been employed for rendering the surface of various films suchas polyolefin films printable have been observed to increase thewettability as measured by contact angle to a degree. How ever, theextent of increase of wettability for certain films especially those ofpolyethylene terephthalate, and 0f the various fluorine-containinghydrocarbon polymers, has not been sufficient to prevent a tendencytoward fogging.

It is an object of this invention, therefore, to provide a convenientand commercially adaptable process for increasing the wettability ofpolymeric films, especially those difficult to wet. The foregoing andrelated objects wil more clearly appear hereinafter.

These objects are realized by the present invention which, brieflystated comprises subjecting the surface of a normally hydrophobic (i.e.,water non-wettable), organic polymeric film to the action of anelectrical discharge having an energy level below 15 electron volts, ina gaseous atmosphere consisting essentially of the vapor of an organicagent selected from the group consisting of acrylic acid and diethylmaleate, whereby to render said surface water wettable to a degree suchthat the contact angle for said surface is less than 15.

A suitable arrangement of apparatus for carrying out the process of thisinvention is illustrated schematically in the accompanying drawing.Referring now, to the single figure of the drawing, a continuousself-supporting film of a particular polymer, e.g. a polymer selectedfrom the group consisting of polyethylene terephthalate, poly- 3,274,091Patented Sept. 20, 1966 vinyl fluoride,tetrafiuoroethylene/hexafiuoropropene copolymer, linear polyethylene andlinear polypropylene, is passed continuously between a set of spacedelectrodes consisting of rotating metal roll 1 which is connectedelectrically to ground and one or more stationary hollow metal tubes, 2which are disposed parallel to the longitudinal axis of the roll andspaced a distance of from 0.005 to 0.25 inch from the surface thereof.The tubes constituting the positive electrode are each connectedelectrically to a suitable power source (for example, a high frequencyspark generator) which supplies an alternating (or pulsating direct)current of the required intensity at the required voltage and frequency.A gaseous atmosphere consisting essentially of the vapor of the treatingagent as the sole active agent and preferably admixed with a suitablecarrier gas such as nitrogen is fed continuously to the hollow interiorof the electrode tubes through. distributor ducts 3 and issues from thetubes through suitable openings therein at the gap between each tube andthe roll. The electrical discharge takes place in the atmospherecontaining the vapors of the organic agent.

A carrying out the surface treatment of this invention, the potentialdifference between the electrodes may vary from very low voltage-s inthe order of volts up to pulsating volts of 100,000 and above. Ingeneral, it is preferred to maintain the voltage in excess of 2000volts. Frequencies from 60 cycles per second up to 500,000 cycles persecond and above can be used. Frequencies in the range of 300,000 to500,000 cycles are preferred in order to obtain effective treatment atcommercially acceptable exposure times. While the current to theelectrodes may range up to 5.5 RF amperes or more, for optimum results arange from 0.3 RF amperes to 2.1 RF amperes is preferred. Power to theelectrodes may range from 10 watts per lineal inch of the electrodelength to 100 watts per lineal inch of electrode length. The electricaldischarge employed herein has an energy level below 15 electron voltsand is not to be confused with high or intermediate energy irradiationsheretofore used in the treatment of polymeric surfaces.

The electrodes are preferably spaced from about 0.01 inch to about 0.125inch. Useful results can be obtained when the electrode gap is as low as0.005 inch to as much as 0.25 inch provided suitable adjustment is suchfeatures as amount of current, electrode dimension and exposure time aremade.

While the surface treatment of this invention will substantially improvethe water Wettability of any organic polymeric film the surface of whichis not readily wetted by Water, i.e., organic polymer film surf-aces onwhich water tends to gather in droplets on the surface of the filmrather than form a continuous film thereover, it is particularlyeffective in enhancing the water-wettability (as indicated by reductionof the contact angle to a value below 15 of the surfaces of organicpolymeric hydrophobic films selected from the group consisting of filmsof polyethylene terephthalate, polyvinyl fluoride,tetrafluoroethylene/hexa fluoropropene copolymer, linear polyethyleneand linear polypropylene.

In order to realize the extremely low contact angles which characterizethe treatment of this invention it is required that the gaseousatmosphere in which the electrical discharge takes place contain as thesole reactive agent a vapor of an organic agent selected from the groupconsisting of acrylic acid and diethyl maleate.

Time of exposure to the electric dis-charge treatment is not especiallycritical and effective treatments are realized at exposure times asshort as 1X10" second,

and no adverse effects are noted at times as long as 60 seconds. Evenlonger exposure times can be employed although, for economic reasons,exposure times as short as possible consistent with eifective treatmentwould normally be employed. Preferably the time of exposure of thepolymeric surface to the electric discharge treatment should be at least4 10- second.

The following examples will serve to more fully illustrate theprinciples and practice of this invention.

EXAMPLE I Polyethylene terephthalate film was passed at the rate of 2.2feet per minute through a 0.026 of an inch gap formed by electrodesarranged as shown in the accompanying drawing. A continuous flow of agaseous mixture consisting of nitrogen saturated with acrylic acidvapors Was fed to the hollow tubes constituting the positive electrodeswhereby to maintain an atmosphere in the gap consisting of acrylic acidvapors as the sole reactive agent. The hollow tubes were connected to aLepel high frequency spark generator (Lepel High Frequency Laboratories,Inc.) and the metal roll was grounded. The current :to the tubeelectrodes was between 1.3 and 1.7 RF amperes at a frequency of about300,000 cycles. The resulting treated film surface has a contact angleof 3 whereas the untreated film surface had a contact angle of 65 andfilm subjected to the same electrical discharge in air had a contactangle of 32. The treated and control films were placed over reservoirsof Warm water. The water condensed on the treated surface to form acontinuous film of water, and consequently the treated film remainedclear and transparent, whereas the controls became 'fogged under thesame treatment.

An additional sample of the polyethylene terephthalate film was treatedin the same fashion, replacing the acrylic acid vapors with vapors ofdiethyl maleate. The contact angle of the resulting treated film surfacemeasured Determination of contact angle Contact angle in thisspecification may be defined as where 0,, is the advancing contact angleand 0, is the receding contact angle. The procedure is as follows:Handling the film only with tweezers, a one-half inch by one-inch sampleis washed briefly in deionized Water and then similarly in methyl ethyl'ketone, followed by drawing in a circulating air oven for about tenminutes at 60 C. After exposing the sample to a radio-active staticeliminator and brushing off any dust with a small camels hair brush itis placed in the center of the specimen plat-form of the contact anglegoniometer. The contact angle goniometer consists essentially of amicroscope mounted with its axis horizontal, equipped with a mechanicalstage (the specimen block) that can be raised and lowered or moved fromside to side. The normal eye piece of the microscope is replaced with aprotractor eye piece which is divided into degrees on a rotating scalewith a vernier in minutes on a fixed arm. The cross-hairs in the eyepiece divide the field of view into quadrants. A drop of deionized wateris pushed onto the film surface from a capillary dropper mounted abovethe stage. The capillary dropper is made from an ordinary eye dropper bydrawing the tip into a one-inch long capillary with a diameter justsmall enough to prevent water from running out of the tube undergravitational force only. T o assist in dispersing liquid from thedropper the tip of the capillary is ground about 30 off theperpendicular. The protractor scale is then revolved until itscross-hair is parallel to the surface on which the drop is resting. Theother cross-hair is adjusted until it is tangent to the drop at thepoint of contact with the surface on which it is resting. The anglebetween the cross-hairs inside the drop is read from the protractorscale. This is the advancing contact angle. Using the capillary dropper,water is subtracted from the drop on the film sample and the recedingcontact angle is recorded. For both advancing and receding contactangles the drop perimeter must move and to insure this the drop isviewed as water is being added or subtracted. Due to water evaporation,an advancing water drop will begin to recede within about 30 secondsafter it has stopped advancing. Therefore, the advancing contact anglemust be measured soon after the drop perimeter has stopped moving. Areceding drop may take as much as 30 seconds to come to equilibriumafter subtraction of :water has stopped. Since water evaporation merelycauses more water loss and does not affect the receding contact angle itis best to wait about 30 seconds before taking this reading.

EXAMPLE II Following the procedure of Example I, samples of Tedlarpolyvinyl fluoride film were subjected to an electrical discharge in anatmosphere consisting of nitrogen saturated with active organic vapor.Acrylic acid treated film exhibited a treated surface contact angle of3, and diethyl maleate, 6. The untreated film had a contact angle of 71and the contact angle of film treated with the electrical discharge inair was 40.

EXAMPLE III Following the procedure of Example I, samples of Teflon FEPtetrafiuoroethylene/hexafiuoropropylene copolymer film were subjected tothe electrical discharge in an atomsphere consisting of nitrogensaturated with active organic vapor. Acrylic acid treated film exhibiteda treated surface contact angle of 3, and diethyl maleate treated filmhad a contact angle of 5. Film treated only with the electricaldischarge in air had a contact angle of 46; and untreated film, acontact angle of 104.

EXAMPLE IV Samples of linear polyethylene film were subjected as inExample I to an electrical discharge in an atmosphere consisting ofnitrogen saturated with active organic vapor. With acrylic acid, thecontact angle of the treated film surface was 3, and with diethylmaleate, 13. The untreated control had a contact angle of 97.

EXAMPLE V Samples of linear polypropylene film were treated as in theprevious examples. Film treated in an atmosphere containing diethylmaleate as the sole active agent exhibited a contact angle of 3. Theuntreated film showed a contact angle of It is obvious from theforegoing description and examples that the process of this inventionprovides a simple and commercially feasible operation which can beutilized for increasing the wetta-bility of normally hydrophobic organicpolymer film surfaces to the degree that essentially no fogging ofsurfaces of these polymeric films in contact with moist atmosphereswould be encountered. The advantage of such a film for the wrapping ofmoist products is readily evident. 'In other applications such as insolar stills the use of film having wettable surfaces is of greatimportance to the efiicient operation of these devices.

I claim:

1. A process for improving the water-wettability of the surface of anormally hydrophobic, organic polymeric film which comprises subjectingthe surface of said film to the action of an electrical discharge havingan energy level below 15 electron volts, in a gaseous atmospheresaturated with, as the sole active ingredient, the vapor of an organicagent selected from the group consisting of acrylic acid and diethylmaleate, whereby to render said surface water wettable to a degree suchthat the contact angle for said surface is less than 15.

if! a 6 2. The process of claim 1 wherein the film is poly- ReferencesCited by the Examiner ethylene terephthalate film. m:

3. The process of claim 1 wherein the film is poly- UNITED STAiLSPATENTS vinyl fl id fil 3,068,510 12/1962 Coleman 204-165 4. The processof claim 1 wherein the film is tetra- 5 FOREIGN PATENTSfluoroethylene/hexafluoropr0pene copolymer film.

5. The process of claim 1 wherein the film is linear polyethylene film.H 1 6. The process of claim 1 wherein the film is linear IO N MACKPlmmry Exammei' polypropylene fil 10 H. S. WILLIAMS, Assistant Examiner.

845,897 8/1960 Great Britain.

1. A PROCESS FOR IMPROVING THE WATER-WETTABILITY OF THE SURFACE OF ANORMALLY HYDROPHOBIC, OR GANIC POLYMERIC FILM WHICH COMPRISES SUBJECTINGTHE SURFACE OF SAID FILM TO THE ACTION OF AN ELECTRICAL DISCHARGE HAVINGAN ENERGY LEVEL BELOW 15 ELECTRON VOLTS, IN A GASEOUS ATMOSPHERESATURATED WITH, AS THE SOLE ACTIVE INGREDIENT, THE VAPOR OF AN ORGANICAGENT SELECTED FROM THE GROUP CONSISTING OF ACRYLIC ACID AND DIETHYLMALEATE, WHEREBY TO RENDER SAID SURFACE WATER WETTABLE TO A DEGREE SUCHTHAT THE CONTACT ANGLE FOR SAID SURFACE IS LESS THAN 15*.